The present invention relates to a stencil printing machine.
There has been known such a stencil sheet as is produced by bonding a resin film and a multi-porous substrate made of a multi-porous sheet with an adhesive.
This stencil sheet is prepared by perforating the film section by the use of a heat source such as a thermal head, and then is wrapped around a printing drum with the multi-porous substrate inside. Then, printing is done by passing ink through the stencil sheet to a printing paper.
In this type of printing using the above-described stencil printing machine, however, the multi-porous substrate itself has low perviousness to ink, resulting in ununiform ink transfer to a printing paper, that is, in an ununiformly printed image. This is because the multi-porous substrate produced of a multi-porous sheet has uneven density and thickness.
To make ink ununiformly transferred to paper look as if uniform, it is necessary to spread ink from the portion where ink has passed into the printing area of the paper, to the portion where no ink has been transferred. Thus spreading ink, however, will result in the presence of an excessive amount of ink within the printed area on the printing paper where ink passes through, and accordingly in ink offset and strike-through.
As a means for improving the ununiform ink condition on paper, there is used only a resin-film stencil for printing, that is, without using the multi-porous sheet substrate. In this case, the stencil sheet can not be conveyed in the machine, and is wrinkled in stencil preparation or when wrapped around the printing drum, and accordingly printed matter faithful to an original master copy is unobtainable. Such a technology has been disclosed in Japanese Patent Laid-Open No. Hei 5-220919.
The stencil sheet produced by bonding a conventional resin film and a porous substrate such as a porous sheet with an adhesive is generally of the order of 30 to 50 .mu.m in thickness.
Therefore, for discharging the stencil sheet of this thickness by means of the stencil discharge section provided within the stencil printing machine, space as this thickness is needed. In this case, however, a limited quantity of discharged stencils is around 20 to 100, albeit it depends on the discharge system and capacity, and the discharge section is not capable of accommodating a large quantity of spent stencils.
Furthermore, the substrate such as the multi-porous sheet of the stencil sheet to be discharged to the stencil discharge section contains a large amount of ink; that is, ink is used wastefully.
By the way, the prior art technique for printing by the use of only a perforated resin-film stencil has been disclosed in Japanese Patents Laid-Open No. Hei 5-309932 and No. Hei 5-318900. According to the technique of Japanese Patent Laid-Open No. Hei 5-309932, a resin film and a substrate that have been separated are stripped off at the stencil discharge apparatus and received in a common receiving box. Therefore, the quantity of stencil paper discharged makes no difference from that of the stencil sheet made by bonding the prior art resin film and a multi-porous substrate of a multi-porous sheet with an adhesive.
According to the technique disclosed in Japanese Patent Laid-Open No. Hei 5-318900, the separated porous substrate is held inside the machine and accordingly the a space large enough to hold the porous substrate within the machine is required.
For printing by the use of a prior art stencil sheet made by bonding a resin film and a multi-porous substrate such as a multi-porous sheet with an adhesive, the stencil sheet is wrapped around the printing drum. At this time, if a perforated resin film which is on the outer side of the stencil sheet is impressed, ink passes through the perforated portion of the resin film, smearing the pressed member. Therefore, the stencil sheet, when wrapped around the printing drum, can not be impressed, and accordingly it is necessary to wrap the stencil sheet around the printing drum by using an impressing member with the printing paper inserted between the printing drum and the impressing member.
Furthermore, for fully impregnating a stencil sheet with ink from a multi-porous substrate such as a multi-porous sheet to a perforated resin film, a considerable pressure and/or impression time are required, resulting in excessive ink transfer to the printing paper used in wrapping the stencil sheet around the printing drum.
When printing is continuously performed immediately after the wrapping of the stencil sheet around the printing drum, the printing paper used in wrapping the stencil sheet is discharged onto a discharge tray, on which succeeding paper is stacked continuously. Since a large amount of ink remains on the paper used in wrapping, ink will transfer to the back side of the paper discharged thereon. The paper used in wrapping is a printed matter not faithful to the original master copy because of presence of such a defect as excessive strike-through. That is, the printing paper is used wastefully.
Furthermore, when the stencil sheet is wrapped around the printing drum with a decreased pressure and/or impression time required for wrapping around the printing drum, the first one to three printing sheets are not fully impregnated with ink, resulting in a failure in producing a printed matter faithful to an original master copy.